Carbodiimides are important reagents in synthetic chemistry and are employed in a wide variety of transformations, in which they generally function as dehydrating agents. Carbodiimides such as dicyclohexylcarbodiimide (DCC), diisopropylcarbodiimide (DIC), and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) are used primarily as peptide coupling reagents, and are employed less frequently for direct O-acylations.
A major problem in the use of carbodiimides such as DCC or DIC has been the frequently encountered difficulty of separating the desired reaction product from the neutral urea formed as a result of the dehydration and still retained with the organic material. EDC and other "water soluble" carbodiimides have addressed this issue, as the resulting urea by-products formed from the dehydration reaction are basic and can be easily removed with a dilute acid wash.
For example, in connection with membranes for solid phase protein sequencing discussed by U.S. Pat. No. 5,011,861, issued Apr. 30, 1991, when nucleophiles having both amino and thiol functions are contacted with a reagent such as PVDF, the thiol functions react to bind the nucleophile to the membrane surface, thereby allowing the amino functions to remain free for the coupling of the peptide or protein to the membrane. This coupling can be achieved by reacting carboxyl groups within the peptide or protein with the amino groups using carbodiimides (noted as being preferably those which are water-soluble). Thus, these carbodiimides reagents are capable of effecting a dehydrative condensation of the reactive amino group of the peptide with the reactive carboxyl group of the mating material in an aqueous solvent.
In addition to protein sequencing, other applications pertaining to protein coupling (formation of peptide bonds) include preparations of peptide-enzyme conjugates (e.g. U.S. Pat. No. 4,517,290, issued May 14, 1985).
Further, carbodiimides have found applications as sources of isoureas for esterification, typically to introduce tert-butyl esters. Many methods have been reported for the synthesis of tert-butyl esters, a number of which rely on strong acids, bases, or expensive and toxic reagents. For example, N,N'-diisopropyl-O-tert-butyl isourea has been the reagent of choice for the introduction of the tert-butyl ester into a variety of N-protected amino acids and derivatives. This reagent gives good yields of tert-butyl esters, but has several drawbacks. The formation of isourea proceeds slowly and in only moderate yield, and requires CuCl to catalyze the conversion. The resulting reaction mixture must be washed, filtered, and distilled to obtain acceptably pure reagent. Removal of the residual N,N'-diisopropylurea formed in the esterification reaction from the target ester can be difficult and chromatography is often necessary.
However, the previously known water soluble carbodiimides typically bear charged or basic amino groups, and their corresponding isoureas are ineffective for esterifications which depend on initial protonation of the isourea moiety. In ester formation, carbodiimides such as DCC and DIC have been used infrequently as direct O-acylating agents, due in part to the formation of undesirable N-acylurea as well as the accompanying urea, which can further complicate purification.
Consequently, new carbodiimides that are highly reactive but which are easy (in their original form or their derivatized form after the reaction) to remove gently are still needed for applications in a variety of organic syntheses.